Design and analysis of inverse tapered silicon nitride waveguide for flat and highly coherent supercontinuum generation in the mid-infrared

We numerically investigate a promising mid-infrared supercontinuum (SC) source through the design of an on-chip complementary metal oxide semiconductor compatible 3-mm-long inverse tapered waveguide made using stoichiometric silicon nitride (Si\(_3\)N\(_4\)) as the core and silica (SiO\(_2\)) glass as both the upper and lower claddings. The proposed waveguide is designed for pumping only in the anomalous dispersion regime. To explore SC generation in terms of spectral flatness and mid-infrared expansion, three different types of inverse tapered geometry are analyzed. These are inverse linear tapering, hyperbolic tapering, and parabolic tapering approaches. The proposed structures are optimized by varying their width based on the tapering variation coefficient along the pulse propagation direction. Keeping the waveguide thickness (height) constant at 2 \(\upmu\)m, each geometry is engineered by varying widths between 1.2 and 0.75 \(\upmu\)m in the mode of inverse tapering approach. Using a pump at 1.55 \(\upmu\)m with a peak power of 8 kW, SC generation is explored in all tapering geometries with an ultrashort pulse of 50 fs. The largest SC spectra covering a region from 0.76 \(\upmu\)m to beyond 6.67 \(\upmu\)m have been realized with a good spectral flatness by the parabolic-type taper among the three tapering geometries considered here. To the best of the authors’ knowledge, this is the first time report of broadband SC coverage through numerical study in the region of mid-infrared by Si\(_3\)N\(_4\) planar design through a tapering approach. After spectral broadening up to 3.5 \(\upmu\)m with a conventional uniform design, a dip has been observed at the middle of the spectra which gradually becomes large with further dimensional variations of a typical uniform waveguide. However, such spectral dip problems can largely be eliminated through selective spectral enhancement by applying an inverse tapering approach. The coherence calculation of the predicted SC coverages also shows a high coherence at the proposed tapers output. The proposed inverse tapered planar designs show a significant improvement not only in mid-infrared spectral expansion but also in spectral flatness compared to the axially unvarying waveguide design.